Recovery of fatty acids from dilute aqueous solutions



Patented Jan. 9, 1951 I RECOVERY OF FATTY ACIDS FROM DILUTE AQUEOUSSOLUTIONS Ralph W. Dornte, Hatboro, Pa., assignor to Standard OilDevelopment Company, a corporation of Delaware Application August 16,1948, Serial No. 44,454

2 Claims. 1

This invention relates to an improved process for extracting anddehydrating fatty acids as found in low concentrations in aqueoussolutions. More particularly, it relates to an efficient commerciallyfeasible process for the extraction and purification of a mixture offatty acids as found in low concentrations in the aqueous layerresulting from hydrocarbon synthesis reactions.

Hydrocarbon synthesis reactions are performed by contacting hydrogen,and oxides of carbon with catalysts under various temperature andpressure conditions. The temperatures employed vary widely, as forexample, in the range from about 400 F. to about 800 F. and aregenerally in the range from 500 to about 700 F. The particulartemperature employed depends upon, among other factors, the type ofnon-gaseous hydrocarbon product desired, the character and the activityof the particular catalyst utilized, the throughput and composition ofthe synthetic gases and upon the reaction pressure. The pressures,likewise, vary considerably and are a function of other operativeconditions such as catalyst employed, activity of the catalyst,character of the feed gases and the temperatures utilized. Operationssuch'as described are generally conducted under conditions to secure themaximum yield of hydrocarbon constituents containing 4 or more carbonatoms in I the molecule. However, under the conditions of the operation,various side reactions occur which result in the production of valuableoxygenated compounds.

The proportion of the type products obtained thus vary with theconditions. In all cases, however, gaseous products removed overheadfrom the reaction zone are condensed and latter factor is involved inthe distribution of the oxygenated compounds in the two phases. Innormal operations, the ratio of water produced to oil produced may varyover the range from about 0.8 to 3.0 volumes of water per volume of oil,depending upon the operating conditions and the catalyst employed duringthe synthesis. Accordingly, there is a wide variation in the proportionof the total oxygenated compounds existing in the water phase? and thisextends over the range from about 10 to 40 weight percent.

The oxygenated compounds found in the water layer comprise the neutralcompounds including alcohols, aldehydes, ketones and esters, and fattyacids.

The neutral oxygenated compounds are recovered from the water layer bydistillation carried on below C. The neutral compounds and theirazeotropes are thus stripped off first, leaving substantially only thefatty acids in the water. The stripped aqueous layer or acid waterbottoms from most synthesis runs contains the C2-C0 aliphatic acids in atotal concentration equivalent to about 2 to 5 weight per cent or aceticacid, often nearer the lower figure. It is desirable to recover theseacids in marketable purities and substantially quantitatively from thewater, since their disposal as polution waste is not practical and anychemical disposal involves expense in the hydrocarbon synthesis process.In addition, these organic acids represent valuable chemical rawmaterials for industry.

Economic recovery of acids in such low concentrations is quite diflicultto achieve by conventional distillation methods because of theprohibitive heat requirements and equipment sizes necessitated by thevery dilute feeds. Difficulties are encountered in extraction processeswhen a single extractive solvent is used due to the azeotroping of thesolvent with the acids present. This is especially true of the higherboiling, i. e. C4 to C6 acids, when polar organic high boilingextractants are used. The consequent difllculty in separation of the C4to Cl Cc acids.

accuse tracted time intervals. This also contributes to solvent lossesby decomposition or polymerimtion reactions.

It has now been found that a two-stage sol-..

vent extraction process employing a high boiling hydrocarbon to extractthe 04 through Cc acids in the first stage and a high boiling polarorganic solvent to remove therernaining lower acids in the second stageis ideally adapted to overcome the before-mentioned diiiiculties.

The liquid hydrocarbons, used in the first stage as stated above,selectively extract the C4 through The term, high boiling hydrocarbontherefore, is relative to the Cs acids. It is to be understood thatwherever the term, high boiling hydrocarbons, is hereafter used, that itconnotes normally liquid hydrocarbons boiling over 205 C. which areimmiscible in water and have good capacities for the acids extracted.Among such hydrocarbons are: sym. triethyl benzene, B. P. 216 0.;naphthalene, B. P. 218" 0.; methyl naphthalene. B. P. 245 C.; diphenyl,B. P. 256 0.; diphenyl methane, B. P. 266- 0.; acenaphthene, B. P. 2780.; dibenzyl, B. P. 284 C.; tetrahydronaphthalene, B. P. 207 C.;hexahydronaphthalene, B. P; 206 C.

The high boiling polar organic solvents can then be used to extract thelower fatty acids such second extractor II. The preferred polar organiccompound used as the extractant is isophorone. The ramnate is fed to thetop of the extractor It at about 170 C. and the isophorone entersnear'the bottom at about 196 0. through line I8. The extracted or spentwater layer, at about 168 0., passes to a solvent recovery still llthrough line H. The isophorone water azeoas acetic and propionic fromthe raillnate of the first extraction. It is to be understood thatwhenever the term, high boiling polar organic solvent, is used hereafterthat it connotes normally liquid organic compounds having at mostlimited solubility in water, good capacities for acetic and propionieacids, and which have boiling points of 160 C. or higher. Among suchpolar solvents are: methyl cyclohexanone, B. P. 165 0.; furfural, B. P.162 0.; isophorone, B. P. 215 C. and the cresols.

The general term, mixture of fatty acids, is hereafter used to indicatea mixture of up to and including the Cs acids.

This invention will be better understood by reference to theaccompanying flow diagram.

In the system shown, the aqueous acid feed containing the mixture offatty acids is fed through line I to the upper part of the first stageextractor 2 and the high boiling hydrocarbon extractant is suppliedthrough line 8 to the lower part of the extractor 2 in which acounter-current now of aqueous acid liquid and extractant occurs toenable the hydrocarbon extractant'to remove the C4 and higher acids. Theacid-enriched extractant leaves the extractor 2 through line 4 andenters dehydration still 5 where water is removed by distillation fromthe extract. Some acid and hydrocarbon are removed with the water takenon overhead through line 6 and condenser 8 to decanter 1. The organiclayer which separates from the water in the decanter is returned tostill 5 by line 9 or reflux, and the water layer with acid is returnedto extractor 2 through line I. Th acid-enriched water-free extractantleaves still 5 through line H and goes to acidsolvent separating stillII. The vaporized acids are taken off overhead through line It andpurifled by fractionation elsewhere. The hydrocarbon extractant left asresidue in still I 2 is returned to extractor 2 through line 3. Freshmake up hydrocarbon oil may be added as needed.

The aqueous raihnate containing Ca and C: acids, with the C4 through Ccacids removed. is discharged from extractor 2 through line H to a trope(B. P. 94 C. at 1 atm. 12.6% isophorone) is then taken overhead throughline I! and condenser 3! to decanter :0 where it separates into twolayers. The water layer is returned through line 2| as reflux to thesolvent recovery still and the isophorone layer is sent to the recycledisophorone stream by line 32.

The acid-enriched polar organic extractant leaves the extractor I!through line 22 to dehydration still 23. Th isophorone water azeotropeis taken overhead from still 23 through line 24 and condenser 32 todecanter 25. The isophorone layer is returned as reflux through line 26and the water layer is returned through line 21 to extractor to recoverentrained acid. The acidenriched water-free extractant leaves still 23through line 28 to acid-from-solvent separating still 29. The acids aretaken of! overhead through line It and purified by fractionationelsewhere. The isophorone is returned to extractor II through line l8.

The operational temperatures on this two-stage consecutive extractionprocess are dependent upon a large number of engineering details suchas: (1) the temperature of the feed, (2) the utilization of heatexchangers, (8) operational pressures which may range from mm. to 30 p.s. i. g. In the main, the temperatures and pressures for this prqocessmay lie within the limits of 25 to 250 C. for temperature and 100 mm. to30 p. s. i. g.

for pressure. The foundamental principle oi the procas; namely, theextraction of the higher boi ug acids with a high-boiling hydrocarbonsolvent and the subsequent extraction of acetic and propionic acids witha high-boiling polar solvent is unaffected by these variations. Thisseparation of the acids in two stages can be accomplished readil withinthe temperature and.

pressure ranges indicated.

The extractive powers of some of the preferred solvents are given in thetables that follow.

The acid distribution constant or acid capacity factor (K1) of thesolvent for acetic acid is the ratio of the acid concentrations (weightper cent) in the solvent phase and in the water phase at equilibrium.The capacity factor primarily determines the ratio of solvent to feed inthe extractor and also the size and number of plates in the extractor.It is desirable for the extracting solvent to have a high capacityfactor to minimize the solvent circulation and hence the size of therecovery plant. For the extraction of acetic acid from water, a solventpossessing a capacity factor equal to or greater than 1 is desirable.

The distribution constant (K2) for water is the ratio of the waterconcentration (weight per cent) in the solvent phase to that in theaqueous phase at equilibrium. The selectivity beta of the solventmeasures the sharpness of separation between water and acetic acid whichcan be accomplished in the extraction process. The selectivity is theratio of the acid distribution constant to he water distributionconstant.

Distribution equilibrium of the fatty acid: be-

tween water and hydro arbon ezm'lctant.

tetrahudronaphthalene' I Organic Layer A ueoulh er Solvent F3 no A010Weightflr x. K. mu

Acid, Weight Per we! a Cent Cent Per t CHiCOOH OHzOOOH 25 1.75 .01 .m.0177 .55 75 .113 1.80 .00 .0181 .30

OaHrCOOH 01810.00]! 25 .(X 1.01 .08 .11174 .0102 72 75 .010 1.09 .66.015 .0110 1 4 'r m dro MB 1 m 14s eo 41 0149 an e mi 0.11.0001!OcfluOOOH 25 .75 1.11 .2! 2.78 .0111 .250 15 .12 .95 .31 2.32 .0011; .m

l-ClHuCOOH t-OeHuOOOH 25 .97 1.10 .11 8.81 .0119 .740 75 .93 1.38 .157.15 .013! .557

x 01110001! weight per cent in organic layer 011 00011 weight per centin aqueous layer K 1190 weight per cent in organic layer H O weightperoentinaqueoullayer Beta-INK:

TABLE II Zquilibrium distr bution 0! jetty acid: between water andsolvent Organic Layer Aqueoul IAyer Temp. Solvent K1 K1 Beta Acid,Weight Acid,Weight Per Can Pu mt Per Cent mt OHzOOOH CHICOOH 25 1.094.25 .80 1.10 1.27 .043 80 75 1.05 0.15 .90 .97 1.10 .063 18 125 1.157.55 1.13 1.02 .133 12 F' mmcoon ccncoon 01110001! 08100011 25 .92 12.071.09 1.04 .54 .123 0.8 75 .92 15.27 LN 3.13 .85 .15 5.8 o-Cresol, B. P.191.5" 0..---. CHICOOH O'mcoon (3111000131 weight percent in organiclayer 011100011 weight per cent in aqueous layer 11 0 weight per cent inorganic layer 11:0 weight per cent in aqueous layer Beta- KII Q Thehydrocarbon solvent extracts only small amounts of the C: and C3 fattyacids, but the extractor can be designed to remove all of thehigher-acids. The distribution constant (K1) of tetrahydronaphthalenefor acetic acid at 25 C. is only 0.0099, whereas the constant forvaleric acid is 2.78 so that for equal concentrations of these acids inthe aqueous layer, the hydrocarbon would extract roughly 200 times asmuch of valeric as acetic acids. The detailed equilibrium distributionresults for tetrahydronaphthalene and decahydronaphthalene with theaqueous fatty acids are given in Table I. It should be noted that theacid distribution constant or capacity factor (K1) 0! the hydrocarbonincreases rapidw with the number of carbon atoms in the fatty out in astandard apparatus for carrying out extractions with appropriateauxiliary stills and separator vessels and at elevated temperatures.Fresh extractant may be added as needed to the recycled and recoveredextractant.

Among the advantages of the process of this invention is that thehold-up time and consequent solvent loss of the polar organic extractantis decreased. The undesirable build-up of the C to Ce acids is alsodecreased while the scope or recoverable products is extended.

Such modifications are part of this invention and are intended to beincluded therein.

What is claimed:

1. A process for separating C4 to Cu fatty acids 10 and C: to C3 fattyacids from a dilute aqueous solution of said acid, which comprises firstextracting the C4-C6 fatty acids from said solution with a high boilingliquid cyclic hydrocarbon extractant to essentially remove the G5 and C5acids 18 which tend to azeotrope with isophorone, said hydrocarbonextractant boiling above 205 C. and essentially higher than the C5 andCa acids, removing the hydrocarbon extractant containing the extractedC4-C5 acids from the residual aqueous solution containing the Ca and C3acids. then extracting the C: and C3 with isophorone from the residualaqueous solution which is freed of the Ca and Co acids, and separatelydistilling the C: and C: acids from the isophorone extractant to obtainseparate pure distillates of the C: and

5 C3 acids.

2. A process as described in claim 1, in which the cyclic hydrocarbonextractant is tetrahydronaphthalene.

RALPH W. DORNTE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,917,660 Martin et al. July 11,1938 2,255,235 Wentworth Sept. 9, 1941 2,275,862 Othmer Mar. 10, 19422,895,010 Othmer Feb. 19, 1946

1. A PROCESS FOR SEPARATING C4 TO C6 FATTY ACIDS AND C2 TO C3 FATTYACIDS FROM A DILUTE AQUEOUS SOLUTION OF SAID ACID, WHICH COMPRISES FIRSTEXTRACTING THE C4-C6 FATTY ACIDS FROM SAID SOLUTION WITH A HIGH BOILINGLIQUID CYCLIC HYDROCARBON EXTRACTANT TO ESSENTIALLY REMOVE THE C5 AND C6ACIDS WHICH TEND TO AZEOTROPE WITH ISOPHORONE, SAID HYDROCARBONEXTRACTANT BOILING ABOVE 205* C. AND ESSENTIALLY HIGHER THAN THE C5 ANDC6 ACIDS, REMOVING THE HYDROCARBON EXTRACTANT CONTAINING THE EXTRACTEDC4-C6 ACIDS FROM THE RESIDUAL AQUEOUS SOLUTION CONTAINING THE C2 AND C3ACIDS, THEN EXTRACTING THE C2 AND CO WITH ISOPHORONE FROM THE RESIDUALAQUEOUS SOLUTION WHICH IS FREED OF THE C5 AND C6 ACIDS, AND SEPARATELYDISTILLING THE C2 AND C3 ACIDS FROM THE ISOPHORONE EXTRACTANT TO OBTAINSEPARATE PURE DISTILLATES OF THE C2 AND C3 ACIDS.